As the chill of winter approaches, there is a subtle change at your local gas station you may not notice—the introduction of winter-blend gasoline. You probably won’t see a change in how your car runs, but you might notice that it costs a little less for a fill-up. So, what’s the difference between summer- and winter-blend gas, how does it affect your car’s engine, and why does it cost less? You’ve got questions, we’ve got answers.

What’s the difference between summer- and winter-blend gas?

Winter-blend gas has a higher Reid Vapor Pressure (RVP), which is a measure of fuel evaporation at a given temperature. This higher RVP is mostly due to a higher percentage of butane in the winter-blend fuel mixture. Butane—which you may also know as lighter fluid—has high volatility and vaporizes at lower temperatures, a helpful combination when you start your car’s engine on a cold winter morning.

Other additives in the winter-blend formulation are there to prevent fuel line freezing and improve engine performance when temperatures drop extremely low.

Why is winter-blend gas less expensive?

There are several factors that contribute to the lower cost of gas during the winter, but the main reasons are lower production costs and the law of supply and demand.

The production of winter-blends includes Butane and other fuel additives that are less expensive than gasoline’s components of summer-blend. Consumers get to reap the benefits of these reduced production costs because they have a direct impact on the prices at the pump. Another compounding factor is a reduction in travel.

People tend to drive less in colder weather. With fewer people on the roads, the demand for fuel goes down, lowering the cost of gas even further. According to AAA, you are likely to see the price of fuel drop somewhere between $0.10 to $0.30 per gallon during the winter months.

When does winter-blend gas start?

Your local gas stations are likely to begin the transition from summer- to winter-blend in September or October. The Environmental Protection Agency (EPA) requires gas stations to sell summer gas until September 15. This means gas stations are allowed to start selling winter gas as early as September 16.

However, most refiners will deplete the remainder of their summer supply before making the switch to winter-blends so it could take a bit longer before you notice a difference at the pumps. The roll out of winter-blend is a trickle effect that begins in northern states and gradually moves south.  However, location and climate patterns could expedite or delay the transition. When springtime rolls around, the process is reversed, with the transition from winter- summer-blend formulation starting in the south and moving north.

Why can’t we use winter-blend gas all year?

Butane is less expensive than other gasoline components, but the reason it benefits your car’s engine in winter months is why it hinders engine efficiency in the summer. Butane and the other additives used to create winter-blends tend to have a lower viscosity and boiling point making them more volatile.

If you were to continue using this blend in warmer weather, you might experience excessive evaporation because of Butane’s volatility causing your engine to experience vapor lock. Symptoms of vapor lock include power loss, stalling and difficulty restarting.

You are more likely to harm the environment using winter-blend during the summer months than you are to experience vapor lock. The likelihood of gas evaporating in your vehicle during the summer months is high regardless of which blend you are using, but winter-blends will evaporate at a faster rate due to their extreme volatility.

When these gasses evaporate inside your vehicle’s fuel system, they generate increased levels of smog, ozone and other emissions that are then released into the environment.

Note: Though rare, there are extreme cases when the EPA will issue an emergency waiver that allows for the use of winter-blends during the summer months. Examples include war, ransomware attacks and natural disasters.

Which is better for my car’s engine?

Although you can use both summer- and winter-blend gas any time of the year without much risk to your engine, their respective formulations are ideal for their corresponding seasons.

Additionally, a higher butane concentration lowers fuel efficiency, as the winter blend contains fewer energy units per gallon than other gasoline components. In other words, the miles per gallon are slightly lower when using winter-blend gas.

The only thing you might notice as gas stations transition from summer- to winter-blend fuel is a slightly lower cost to fill up your car’s gas tank. But the science backs up its importance as temperatures drop and your car engine’s needs change.

What is the point of changing back and forth?

In 1970, the EPA established The Clean Air Act. This act is designed to improve the earth’s air quality through the regulation of harmful emissions like ozone, smog, carbon monoxide and other pollutants. The changeover from summer-blend to winter-blend is a seasonal strategy for emissions reduction.

By reducing these air pollutants and improving the overall air quality, people breathe less pollution, are less likely to suffer a premature death or lasting health issues like Chronic Bronchitis and Asthma. Additionally, clean air leads to healthier forests, fewer toxins in food supplies and better visibility conditions.

As temperatures drop, your car’s engine benefits from winter-blend gasoline, designed to perform better in colder conditions while saving you money at the pump.

The seasonal switch may be subtle, but it helps reduce emissions and keeps your vehicle running smoothly during the winter months. Keep an eye out for lower fuel costs as you fill up and enjoy the benefits of winter-blend gas!